The present invention relates to a remote enquiry system using telephone transmission lines, and more particularly to a dial pulse detector for use in such a system.
It is sometimes desirable to remotely control electrical apparatus or make automated enquiry of the status of a given condition using ordinary telephone lines and controlling signals originated by the operation of the telephone instrument itself. When using the so-called multifrequency, pushbutton or Touch Tone type telephone instrument, this can be done relatively easily by simply arranging a series of band pass filters at the receiving end to decode the different frequency signals and convert them into digital or decimal units. However, when using a dial type telephone instrument, signals are generated by the making and breaking of the switch in the dial telephone. These signals, after amplification in the telephone transmission system, appear at the receiving end as a series of spike pulses representing the closing and opening of the switch at the dial instrument. Unfortunately, noise pulses on the transmission line may exceed in amplitude the actual pulses produced by the dial instrument. This makes the interpretation and reconstruction of the dial telephone pulses extremely difficult.
Several types of prior art systems have attempted to reconstruct the signal information supplied by the dial telephone pulses by establishing amplitude threshhold levels which discriminate out the noise. Typically, such systems rely on the assumption that once a telephone connection is established, the amplitude of the dial pulse information signals will exceed the amplitude of the noise signals. See, for example, U.S. Pat. No. 3,927,264. Unfortunately, this is not always true and not only the amplitude of the pulses in a given connection may vary, but even the amplitudes of the pulses in the same train of pulses may vary. Furthermore, the degree and manner of such variations may be drastically different from one pulse train to another. For certain forms of input pulse trains at the receiving end it is, therefore, not possible to choose a proper percentage of the peak pulse amplitude to discriminate out noises while receiving the weak information signals.
One type of prior art system attempts to overcome this problem by using a timing "window" circuit which relies on the fact that the pulse generated by the breaking of the dial telephone switch contacts will be followed by a pulse representing the making of the same switch contacts within a predetermined period of time which is established by the telephone company standards. This type of prior art system is described in U.S. Pat. Nos. 3,879,583 and 3,935,392. The system utilizes logic elements to only count pairs of pulses which occur in a predetermined planned relationship to each other. The defect of prior art systems of this type is that they assume that no noise pulses of any significance will occur between a valid pair of pulses. If such a noise pulse does occur during the break time in such systems, they will miscount the pulses. This situation might occur for trailing pulses of a pulse train due to reduced signal level. Furthermore, the systems described in U.S. Pat. Nos. 3,879,583 and 3,935,392 contain no provision to verify the time requirements between successive breaks of the dial type switch, nor are any means provided for detecting the end of the given pulse train.